Objective To investigate the relationship between adenosine triphosphate-binding cassette superfamily G member 2 (ABCG2) mRNA and protein expressions and chemotherapeutic efficacy and drug resistance in breast cancer patients. Methods 24 patients with breast cancer were selected as study subjects, all patients were treated with anthracycline chemotherapy after chemotherapy, according to WHO criteria, the patients were divided into effective group, stable group and advanced group according to chemotherapy effect. The levels of ABCG2 mRNA and protein in the breast tissue were detected by qPCR and ELISA before and after chemotherapy. Results After chemotherapy, the total effective rate was 41.67%, 58.33% of patients still developed drug resistance. The expressions of ABCG2 mRNA and protein in patients after chemotherapy were significantly higher than those before chemotherapy (P<0.05). The expressions of ABCG2 mRNA and protein were significantly lower in the effective group than in the stable group and advanced group (P<0.05). The expressions of ABCG2 mRNA and protein in the stable group was significantly lower than that in the advanced group (P<0.05). Conclusion The expression level of ABCG2 was correlated with the chemotherapeutic efficacy and drug resistance of breast cancer patients. The higher the ABCG2 expression level, the stronger the ability of promoting the excretion of drugs, the more significant the drug resistance of tumor cells and the worsen the chemotherapy effect.